Determining and Managing Critical Load and Heat Load in the Data Center

Sizing the electrical service for a data center or data room requires an understanding of the amount of electricity required by the cooling system, the UPS system, and the critical IT loads.  The power requirements of these elements may vary greatly from each other, but can be precisely estimated using simple rules once the power requirements of the planned IT load are determined.  Apart from estimating the size of the electrical service, these elements can be used to estimate the power output capacity of a standby generator system, if one is required for the data center loads.

A proper planning exercise in developing a data center, from a single rack sized environment to a full scale data center begins with determining the size of the critical load that must be served and protected.  The critical load is all of the IT hardware components that make up the IT business architecture:  servers, routers, computers, storage devices, telecommunications equipment, etc., as well as the security systems, fire and monitoring systems that protect them.  The process of determining critical load begins with a list of all such devices, with their nameplate power rating, their voltage requirements, and whether they are single phase or three phase devices.  The nameplate information must then be adjusted to reflect the true anticipated load.

Determining the critical heat load starts with the identification of the equipment to be deployed within the space. However, this is only part of the entire heat load of the environment. Additionally, the lighting, people, and heat conducted from the surrounding spaces will also contribute to the overall heat load. As a very general principal, estimate no less than 1-ton (12,000 BTU/Hr / 3,516 watts) per 400 square-feet of IT equipment floor space.

The equipment heat load can be obtained by identifying the current requirements for each piece of equipment and multiplying it by the operating voltage (for all single phase equipment). The number derived is the maximum draw or nameplate rating of the equipment. In reality, the equipment will only draw between 40% and 60% of its nameplate rating in a steady-state operating condition. For this reason, solely utilizing the nameplate rating will yield an over inflated load requirement. Designing the cooling system to these parameters will be cost prohibitive. An effort is underway for manufacturers to provide typical load rating of all pieces of equipment to simplify power and cooling design.

The equipment that will occupy a space has not been determined prior to the commencement of cooling systems design. In this case, the experience of the designer is vital. PTS maintains an expert knowledge of the typical load profile for various application and equipment deployments. For this reason, as well as consideration of future growth factors it may be easier to define the load in terms of an anticipated standard for a given area. The old standard used to be a watts-per-square foot definition.

The nameplate power requirements are the worst-case power consumption numbers required by Underwriter’s Laboratory and in almost all cases, are well above the expected operating power level.  Studies conducted by reputable consulting engineering firms and power supply manufacturers indicate that the nameplate rating of most IT devices is well in excess of the actual running load by a factor of at least 33%.  The U.S. National Electrical Code (NEC) and similar worldwide regulatory bodies also recognize this fact and allow electrical system planners to add up nameplate data for expected loads and multiply by a diversity factor, anticipating that not all devices are running at full load 100% of the time. Calculators gather power consumption data from a wide range of manufactures and further specify various equipment configurations.

Determining the electrical power required to support and cool the critical load within the data center is essential in planning for the development of a facility that will meet the end user’s availability expectations.  This will help specify the size of the Data center physical infrastructure components that will achieve the availability determined by the needs assessment.  Once the sizing determination is complete, conceptual and detailed planning can go forward with the assistance of a competent DCPI systems supplier or, in the case of larger scale data centers, a consulting engineer.

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